I don't think that moderators will be willing to take action and lock/trash announcement threads because of:
- bots and shills
- % of deleted posts
- banned accounts
- pyramid quotes and fake conversations
As you said, it could be used as a tactic to damage/undermine legitimate projects and because we can't prove that these accounts have any direct connection with the project itself, although they bump threads with spam and copypasta, mods can't do anything other than delete posts and ban individual accounts reported for plagiarism...this is at least what I noticed so far.
Ah, but both the Neogame.io and Neluns threads are self-moderated... Like I (successfully) argued for the locking of the Roomdao ANN thread, if there is that much spamming/plagiarism going on - where conversations are literally recycled from just a few days ago - then the OP can and should be held responsible.